Problem: $-2df + 7e + 9f + 2 = -2e + 2f - 8$ Solve for $d$.
Combine constant terms on the right. $-2df + 7e + 9f + {2} = -2e + 2f - {8}$ $-2df + 7e + 9f = -2e + 2f - {10}$ Combine $f$ terms on the right. $-2df + 7e + {9f} = -2e + {2f} - 10$ $-2df + 7e = -2e - {7f} - 10$ Combine $e$ terms on the right. $-2df + {7e} = -{2e} - 7f - 10$ $-2df = -{9e} - 7f - 10$ Isolate $d$ $-{2}d{f} = -9e - 7f - 10$ $d = \dfrac{ -9e - 7f - 10 }{ -{2f} }$ Swap the signs so the denominator isn't negative. $d = \dfrac{ {9}e + {7}f + {10} }{ {2f} }$